Method and Apparatus for Flight Control of Tiltrotor Aircraft

ABSTRACT

A method and apparatus provide for automatically controlling the flight of a tiltrotor aircraft while the aircraft is in flight that is at least partially rotor-borne. The method and apparatus provide for automatically tilting nacelles in response to a longitudinal-velocity control signal so as to produce a longitudinal thrust-vector component for controlling longitudinal velocity of the aircraft. Simultaneously, cyclic swashplate controls are automatically actuated so as to maintain the fuselage in a desired pitch attitude. The method and apparatus also provide for automatically actuating the cyclic swashplate controls for each rotor in response to a lateral-velocity control signal so as to produce a lateral thrust-vector component for controlling lateral velocity of the aircraft. Simultaneously, collective swashplate controls for each rotor are automatically actuated so as to maintain the fuselage in a desired roll attitude. The method and apparatus provide for yaw control through differential longitudinal thrust produced by tilting the nacelles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of copending U.S. patentapplication entitled “Method and Apparatus for Flight Control ofTiltrotor Aircraft,” filed on 5 Jan. 2007, which is a 371 ofInternational Application PCT/US04/24431, filed on 29 Jul. 2004, all ofwhich are incorporated herein by reference for all purposes.

BACKGROUND Field of the Invention

The present invention relates in general to the field of flight controlof aircraft. In particular, the present invention relates to apparatusand methods for controlling the flight of a tiltrotor aircraft.

Description of Related Art

A rotary wing aircraft, such as a helicopter or the tiltrotor aircraft11 shown in FIG. 1, produces lift with at least one main rotor 13, whichcomprises multiple wings, or blades 15, attached to a rotating hub 17.Each blade 15 has an airfoil cross-section, and lift is produced bymoving blades 15 in a circular path as hub 17 rotates. As shown in thefigures, the left and right sides of aircraft 11 are generally mirrorimages of each other, having corresponding components on each side ofaircraft 11. As described herein, a single reference number may be usedto refer to both left and right (as viewed if seated in the aircraft)components when the description applies to both components. Specificreference numbers are used for clarity to refer to specific left orright components when the description is specific to either the left orright component. For example, “rotor 13” may be used in descriptions ofboth the left rotor and the right rotor, and “rotor 13A” and “rotor 13B”may be used in descriptions that are specific to the left and rightrotors, respectively.

The amount of lift produced can be varied by changing the angle ofattack, or pitch, of blades 15 or the speed of blades 15, though thespeed of rotor 13 is usually controlled by use of a RPM governor towithin a narrow range for optimizing performance. Varying the pitch foreach blade 15 requires a complex mechanical system, which is typicallyaccomplished using a swashplate assembly (not shown) located on each hub17.

Each swashplate assembly has two primary roles: (1) under the directionof the collective control, each swashplate assembly changes the pitch ofblades 15 on the corresponding rotor 13 simultaneously, which increasesor decreases the lift that each rotor 13 supplies to aircraft 11,increasing or decreasing each thrust vector 19 for causing aircraft 11to gain or lose altitude; and (2) under the direction of the cycliccontrol, each swashplate assembly changes the angle of blades 15 on thecorresponding rotor 13 individually as they move with hub 17, creating amoment in a generally horizontal direction, as indicated by arrows 21,for causing aircraft 11 to move in any direction around a horizontal360-degree circle, including forward, backward, left and right.

Typically, the collective blade pitch is controlled by a lever that thepilot can move up or down, whereas the cyclic blade pitch is controlledby a control stick that the pilot moves in the direction of desiredmovement of the aircraft. The collective control raises the entireswashplate assembly as a unit, changing the pitch of blades 15 by thesame amount throughout the rotation of hub 17. The cyclic control tiltsthe swashplate assembly, causing the angle of attack of blades 15 tovary as hub 17 rotates. This has the effect of changing the pitch ofblades 15 unevenly depending on where they are in the rotation, causingblades 15 to have a greater angle of attack, and therefore more lift, onone side of the rotation, and a lesser angle of attack, and thereforeless lift, on the opposite side of the rotation. The unbalanced liftcreates a moment that causes the pitch or roll attitude of aircraft 11to change, which rotates the thrust vectors and causes aircraft 11 tomove longitudinally or laterally.

A tiltrotor aircraft, such as aircraft 11, also has movable nacelles 23that are mounted to the outer ends of each fixed wing 25. Nacelles 23can be selectively rotated, as indicated by arrows 27, to any pointbetween a generally vertical orientation, as is shown in FIG. 1,corresponding to a “helicopter mode” for rotor-borne flight using blades15 to provide lift, and a horizontal orientation, corresponding to an“airplane mode” for forward flight using fixed wings 25 to produce lift.Aircraft 11 may also operate in partial helicopter mode at low speeds,in which rotors 13 and fixed wings 25 both provide part of the requiredlift for flight. The operation of aircraft 11 typically includes avertical or short takeoff, a transition from helicopter mode to airplanemode for forward flight, and then a transition back to helicopter modefor a vertical or short landing.

Due to the many variables involved in the control of flight of atiltrotor aircraft, a computer-controlled flight control system (FCS) 28automates many of the functions required for safe, efficient operation.FCS 28 actuates flight-control components of aircraft 11 in response tocontrol inputs generated by one or more of the following: (1) anon-board pilot; (2) a pilot located remote from the aircraft, as with anunmanned aerial vehicle (UAV); (3) a partially autonomous system, suchas an auto-pilot; and (4) a fully autonomous system, such as in an UAVoperating in a fully autonomous manner. FCS 28 is provided withsoftware-implemented flight control methods for generating responses tothese control inputs that are appropriate to a particular flight regime.

In the automatic control methods of current tiltrotor aircraft, when acommand for a change in longitudinal velocity is received by FCS 28while aircraft 11 is in full or partial helicopter mode, FCS 28 induceslongitudinal acceleration of aircraft 11 by changing the pitch attitudeof aircraft 11 to direct thrust vectors 19 forward or rearward. Thechange of pitch attitude is accomplished by FCS 28 commanding theswashplates to tilt forward or rearward using cyclic control, whichcauses aircraft 11 to pitch downward in the direction that the aircraftis commanded to fly. For example, when aircraft 11 is commanded by apilot to fly in the forward direction by moving the cyclic controlforward, FCS 28 commands the swashplate for each rotor 13 to tiltforward, and rotors 13 create a forward pitch moment. As shown in FIG.2, the moment causes the plane of blades 15 to tilt forward and alsopitches aircraft 11 in the nose-down direction, which is visible incomparison to ground 29. Thrust vectors 19 are thus rotated toward theforward direction, and the result is movement in the direction shown byarrow 30.

There are several undesirable influences on aircraft 11 using thisflight control method, especially in a gusty or windy environment. Whenthe pitch attitude of aircraft 11 is changed due to a command to move inthe forward/rearward direction, there is a change in the angle of attackof wings 25 and a corresponding reduction in lift produced by wings 25,and this may produce an undesirable change in the vertical velocityand/or altitude of aircraft 11, which must be countered by changing thevertical climb command. This pitch-attitude-to-vertical-velocitycoupling is especially true when hovering or in a low-speed flightcondition, and is more pronounced in the presence of a headwind. Usingthe current automatic flight control method in this situation, aircraft11 cannot accelerate in the forward direction without a nose-down pitchattitude, and the resulting uncommanded and unwanted vertical motioninterferes with the precise vertical control of aircraft 11.

In the automatic control methods of current tiltrotor aircraft, when acommand for a change in lateral velocity is received by FCS 28 while theaircraft is in full or partial helicopter mode, FCS 28 induces lateralacceleration of aircraft 11 by changing the roll attitude of aircraft 11to direct thrust vectors 19 to the left or right. This is accomplishedusing differential collective blade pitch control, which causes fuselage23 to tilt right or left in the direction that aircraft 11 is commandedto fly. For example, when aircraft 11 is commanded to fly to the right,FCS 28 commands the collective controls on rotors 13 such that rightrotor 13 produces less lift than that being produced by left rotor 13.The resulting thrust imbalance causes aircraft 11 to roll to the right,as shown in FIG. 3, directing thrust vectors 19 to the right and causingaircraft 11 to move in the direction of arrow 31.

This automatic flight control method of tilting aircraft 11 duringlateral maneuvering also causes several problems. When aircraft 11 isoperating in the area of ground effects, which it must do each time itis in close proximity to a large surface, such as ground 29 duringtakeoff and landing, the rolling of aircraft 11 will cause one rotor 13to be closer to ground 29 than the other rotor 13. This difference inrelation to ground 29 will cause the ground effects to be greater on oneside of aircraft 11 than on the other, which will cause the lift of eachrotor 13 to change differently. This difference will cause an additionalroll moment on aircraft 11, and this interferes with the precise controlof aircraft 11. The rolling of aircraft 11 also tends to blow the aircushion out from under one side of aircraft 11, further degrading thecontrollability.

When aircraft 11 is moving laterally, or is hovering in a sideward wind,and wings 25 are tilted to the left or right, there is more drag or windresistance. There is also an increase in down loading, which is theloading of the top of wings 25 by the dynamic pressure caused by rotors13 and the lateral aircraft velocity. Both of these conditions degradethe controllability in the lateral and vertical axes and require morepower than flying level in the same wind conditions.

Aircraft 11 is also subject to upsets from wind gusts, with wind fromany direction causing large position displacements when using thecurrent control methods. For example, if aircraft 11 experiences a windgust from the left side, aircraft 11 will roll to the right. Whenaircraft 11 rolls to the right, thrust vectors 19 are also rotated tothe right, which makes the lateral velocity of aircraft 11 increase tothe right. In current tiltrotor aircraft, if FCS 28 is programmed tohold the aircraft over a specified point on the ground, FCS 28 willcommand aircraft 11 to roll back to the left, causing thrust vectors 19to oppose the gust and to move aircraft 11 back to the position itoccupied before the gust. This method of control has the disadvantage ofallowing the gust to displace aircraft 11 a significant distance fromits original position before FCS 28 can drive aircraft 11 back to theoriginal position.

Other problems with the current methods of control include high responsetime to FCS commands and reduced passenger comfort. Response time toforward and lateral velocity commands is high due to the requirementthat the attitude of aircraft 11 change for these commands to beexecuted, and the high inertia of a large, manned tiltrotor, such asaircraft 11, translates into low response frequencies of the system. Asignificant disadvantage for tiltrotors used to carry passengers is thatpassenger comfort is compromised by tilting fuselage 23 of aircraft 11while maneuvering while hovering or in low-speed flight, such as whileapproaching for a landing and when moving aircraft 11 into position toaccelerate to forward flight.

In the automatic control methods of current tiltrotor aircraft, when acommand to change the yaw velocity (i.e., the velocity of change ofheading) of aircraft 11 is received by FCS 28 while the aircraft is infull or partial helicopter mode, FCS 28 induces a yawing moment usingdifferential longitudinal cyclic control. For example, when aircraft 11is commanded to yaw to the left, such as when a pilot depresses the leftrudder pedal, FCS 28 commands the swashplate for right rotor 13B to tiltforward and commands the swashplate of left rotor 13A to tilt rearward.As shown in FIG. 4, the planes of blades 15A and 15B and the directionof thrust vectors 19A, 19B are tilted in opposite directions, withvector 19A having a rearward thrust component and vector 19B havingforward thrust component. Thrust vectors 19A, 19B create a yaw moment,resulting in rotation of aircraft 11 generally about a vertical yaw axis32 in the direction shown by arrow 33.

DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the application are setforth in the appended claims. However, the application itself, as wellas a preferred mode of use, and further objectives and advantagesthereof, will, best be understood by reference to the following detaileddescription when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a perspective view of a prior-art tiltrotor aircraft;

FIG. 2 is a side view of the tiltrotor aircraft of FIG. 1 executing acommand to fly forward using a prior-art control method;

FIG. 3 is a front view of the tiltrotor aircraft of FIG. 1 executing acommand to fly to the right using a prior-art control method;

FIG. 4 is a side view of the tiltrotor aircraft of FIG. 1 executing acommand to yaw to the left using a prior-art control method;

FIG. 5 is a side view of a tiltrotor aircraft using apparatus andcontrol methods according to the present invention to maintain positionin a hover;

FIG. 6 is a side view of the tiltrotor aircraft of FIG. 4 executing acommand to fly forward using a control method according to the presentinvention;

FIG. 7 is a front view of the tiltrotor aircraft of FIG. 4 using acontrol method according to the present invention to maintain positionin a hover;

FIG. 8 is a front view of the tiltrotor aircraft of FIG. 4 executing acommand to fly to the right using a control method according to thepresent invention;

FIG. 9 is a side view of the tiltrotor aircraft of FIG. 4 executing acommand to yaw to the left using a control method according to thepresent invention;

FIG. 10 is a perspective view of an unmanned tiltrotor aircraftaccording to the present invention; and

FIG. 11 is a perspective view of a civilian passenger version of atiltrotor aircraft according to the present invention.

While the system and method of the present application is susceptible tovarious modifications and alternative forms, specific embodimentsthereof have been shown by way of example in the drawings and are hereindescribed in detail. It should be understood, however, that thedescription herein of specific embodiments is not intended to limit theapplication to the particular embodiment disclosed, but on the contrary,the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the process of thepresent application as defined by the appended claims.

DETAILED DESCRIPTION

Illustrative embodiments of the preferred embodiment are describedbelow. In the interest of clarity, not all features, of an actualimplementation are described in this specification. It will of course beappreciated that in the development of any such actual embodiment,numerous implementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

There is a need for an improved apparatus and improved methods forcontrolling tiltrotor aircraft with minimized tilting of the fuselage ofthe aircraft and enhanced accuracy of control.

Therefore, it is an object of the present invention to provide animproved apparatus and improved methods for controlling tiltrotoraircraft.

The present invention provides a flight control system (FCS)implementing the control methods of the invention for automatic flightcontrol of a tiltrotor aircraft while operating at low airspeeds or in ahover, especially during operation in gusty and turbulent windconditions. In response to a control input for a change in longitudinalvelocity, such as a pilot pushing forward on the cyclic control, the FCScommands the nacelles to rotate in the same direction for directingthrust vectors of the rotors in a longitudinal direction.Simultaneously, the FCS automatically holds the fuselage at a desiredpitch attitude by use of the longitudinal cyclic swashplate controls.

In response to a control input for a change in lateral velocity, such asa pilot pushing sideways on the cyclic control, the FCS commands thelateral cyclic swashplate controls for directing thrust vectors of therotors in a lateral direction. Simultaneously, the FCS automaticallyholds the fuselage to a desired roll attitude by differential use ofrotor collective controls.

In response to a control input for a change of yaw velocity, such as apilot depressing a rudder pedal, the FCS commands the nacelles to rotatefor directing thrust vectors of the rotors in different directions,creating a moment that causes the aircraft to yaw.

The present invention provides significant advantages over the priorart, including: (1) providing longitudinal and lateral velocity controlwhile maintaining the fuselage in a desired attitude; (2) reducingresponse time to forward and lateral velocity commands; (3) increasingaccuracy of aircraft control; (4) reducing position displacements causedby wind gusts; (5) reducing the pitch-attitude to vertical-velocitycoupling; (6) reducing the responses to ground effects; and (7) reducingthe power required for lateral flight.

Referring now to FIG. 5, a tiltrotor aircraft 34 is depicted in a hoverabove ground 35. Aircraft 34 is constructed in the same manner asaircraft 11, described above, but the flight control system (FCS) 36 inaircraft 34 uses the control methods of the present invention toautomatically control the flight of aircraft 34 in response to controlinputs by a pilot or electronic system. Rotors 37, comprising hub 39 andmultiple blades 41, are powered by engines carried within nacelles 43.Nacelles 43 are rotatably mounted to the outer ends of wings 45, andwings 45 are affixed to fuselage 47. As described above, the pitch ofeach blade 41 is controlled by collective and cyclic swashplate controls(not shown) located within hub 39. As described herein, a singlereference number may be used to refer to both left and right components(as viewed when seated in the aircraft) when the description applies toboth components. Specific reference numbers are used for clarity torefer to specific left or right components when the description isspecific to either the left or right component.

In the method of the present invention, a control input for a change inlongitudinal velocity, such as a pilot pushing forward or pullingrearward on the cyclic control, causes FCS 36 to command nacelles 43 torotate in the same direction for directing thrust vectors 49 of rotors37 in a longitudinal direction. Simultaneously, FCS 36 automaticallyholds the pitch attitude of fuselage 47 to a desired pitch attitude,which may be a generally level pitch attitude, by use of thelongitudinal cyclic swashplate controls. For example, FIG. 6 showsaircraft 34 configured for forward motion, with nacelles 43 tiltedforward to give each thrust vector 49 a forward vector component. Thesecomponents tends to drive aircraft 34 forward in the direction shown byarrow 51, while the swashplate controls in each rotor 37 are used tocontrol the pitch attitude of fuselage 47. In addition to a response toa control input, FCS 36 can generate commands in response to alongitudinal position error, in which nacelles 43 are commanded so as toreturn aircraft 34 to a previous position or to fly to a selectedposition.

This longitudinal velocity control method differs from the prior-artcontrol method in that change of the pitch attitude of fuselage 47 isnot required to change the longitudinal velocity of aircraft 34.Maintaining a generally level pitch attitude prevents the angle ofattack for wings 45 from changing and prevents the undesirable change invertical forces that cause problems in controlling the vertical aircraftposition using the prior-art control methods. Specifically, whenhovering or in a low-speed flight condition, especially in the presenceof a headwind, the longitudinal velocity control method of the presentinvention will reduce the pitch-attitude to vertical-velocity couplingby allowing aircraft 34 to accelerate in the forward direction without anose-down pitch attitude. In addition, the method of the presentinvention allows the attitude of aircraft 34 to be controlled to themost favorable condition during the conversion from helicopter mode toairplane mode.

The control methods of the present invention also include an improvedmethod of lateral velocity control of aircraft 34, the method beingimplemented in FCS 36. Aircraft 34 is shown in a hover above ground 35in FIG. 7, with the left rotor labeled as 37A and the right rotorlabeled as 37B. Each rotor 37A, 37B produces a vertical thrust vector49A, 49B, respectively, for lifting aircraft 34. In response to acontrol input for a change in lateral velocity, such as a pilot pushingsideways on the cyclic control, FCS 36 commands the lateral cyclicswashplate controls for directing thrust vectors 49A, 49B of rotors 37A,37B in a lateral direction. Simultaneously, FCS 36 automatically holdsthe roll attitude of fuselage 47 in a desired roll attitude, which maybe a generally level roll attitude, by differential use of rotorcollective controls. In addition to a response to a control input, FCS36 can generate commands in response to a lateral position error, inwhich the lateral cyclic swashplate controls are commanded so as toreturn aircraft 34 to a previous position or to fly to a selectedposition.

For example, FIG. 8 shows aircraft configured for movement to the right(as viewed if seated in the aircraft). When command to move to theright, swashplate controls tilt the plane of rotors 37A, 37B to theright, causing thrust vectors 49A, 49B to have a horizontal component tothe right, and this vector component causes aircraft 34 to move in thedirection shown by arrow 53. While the cyclic swashplate controls inducesideward movement, the differential collective blade control is used tohold the aircraft level, meaning that the collective controls for rotors37A, 37B are actuated independently from each other to maintain thedesired fuselage attitude. This combination of controls allows aircraft34 to move laterally in a stable and precise manner while holdingaircraft 34 in a level roll attitude. A key advantage to the controlmethod of the present invention is that holding fuselage 47 in a levelattitude during lateral flight minimizes ground-effect problems and wingdown-loading problems encountered when rolling aircraft 34 using theprior-art method.

Additionally, the lateral velocity control method of the inventionprovides for improved lateral gust response, which may be reduced by asmuch as around 80%. When a lateral gust hits aircraft 34, FCS 36 willimmediately command the lateral cyclic swashplate control in thedirection opposing the gust while the differential collective bladecontrol is commanded to hold aircraft 34 level. Aircraft 34 will stillhave a tendency to roll with the gust, but thrust vectors 49A, 49B canquickly be redirected to oppose the gust without the need to rollaircraft 34 beyond the amount required to bring aircraft 34 back to agenerally level roll attitude or other desired roll attitude. Asdescribed above, FCS 36 may also generate commands to the cyclicswashplate controls in response to a lateral position error forreturning aircraft 34 to the position aircraft 34 occupied prior to thedisplacement caused by the gust.

The swashplate cyclic controls are limited by physical constraints andthe geometry of the system, such that there is a limited amount of totalcyclic allowed for all cyclic command inputs. The total cyclic used atany one time is the square root of the sum of the squares of thelongitudinal cyclic and the lateral cyclic. As described above, themethods of the invention include using longitudinal cyclic controls forcontrolling the aircraft pitch attitude and using lateral cycliccontrols for controlling the lateral velocity of the aircraft.Longitudinal cyclic is also required to control the aircraft pitchmoment as the location of the center of gravity of aircraft 34 changes.To reduce the total cyclic swashplate commands, the present inventionalso includes a control method for controlling yaw in aircraft 34without the requirement of using longitudinal cyclic controls.

The yaw control method provides for differential nacelle control, inwhich nacelles 43 of aircraft 34 are rotated independently to directtheir thrust vectors 49 in different directions, creating a yaw moment.For example, FIG. 9 shows aircraft 34 configured for yawing in adirection with the nose of aircraft 34 moving to the left (as viewed ifseated in the aircraft). Left nacelle 43A has been rotated rearward, andright nacelle 43B has been rotated forward, directing thrust vectors 49Aand 49B in different directions. Thrust vector 49A has a longitudinalthrust component pointing toward the rear of aircraft 34, and thrustvector 49B has a longitudinal thrust component pointing toward the frontof aircraft 34. This longitudinal thrust differential creates a yawmoment, causing aircraft 34 to rotate in the direction of arrow 55 abouta yaw axis 57. An advantage of this yaw control method is that removingthe yaw control commands from the total cyclic commands provides formore cyclic control range to be available for control of pitch attitude,center-of-gravity changes, and lateral aircraft velocity control. Thisallows for increased longitudinal center-of=gravity range, increasedcapability to hover in a crosswind, increased maneuver envelope for thepitch, roll, and yaw axes, reduced rotor flapping, and simplifiedprioritization of cyclic commands. Also, the yaw control is not limitedby cyclic authority limits.

While shown in FIGS. 5-9 as used with a manned, military-style aircraft34, the improved FCS and control methods of the present invention mayalso be applied to control any type of tiltrotor aircraft. FIG. 10 showsan unmanned aerial vehicle 59 (UAV) constructed as a tiltrotor aircraft.The enhanced accuracy of control permitted by the methods of the presentinvention is especially beneficial with the remote and often automatedoperation of UAVs. Specific functions that are enabled or enhancedinclude automatic launch and automatic recovery from a secondaryvehicle, such as from the deck of a ship at sea, and maneuvering arounda particular location or target in windy conditions with the requiredaccuracy. Also, the reduced response time to forward and lateralvelocity commands provides for a greater maneuver bandwidth, which is agreat advantage for automatically controlled aircraft.

A civilian passenger version of a tiltrotor aircraft 61 is depicted inFIG. 11. As discussed above, the advantages realized from using thecontrol methods of the invention include improved passenger comfort. Byholding aircraft 61 in generally level pitch and roll attitudes whilemaneuvering in hover or low-speed flight, the passengers aboard aircraft57 are not subjected to the tilting and associated change of relativedirection of acceleration due to gravity, or g-forces, felt when usingthe prior-art methods of control.

The present invention provides significant advantages over the priorart, including: (1) providing longitudinal and lateral velocity controlwhile maintaining the fuselage in a desired attitude; (2) reducingresponse time to forward and lateral velocity commands; (3) increasingaccuracy of aircraft control; (4) reducing position displacements causedby wind gusts; (5) reducing the pitch-attitude to vertical-velocitycoupling; (6) reducing the responses to ground effects; and (7) reducingthe power required for lateral flight.

The particular embodiments disclosed above are illustrative only, as theapplication may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. It is therefore evident that the particularembodiments disclosed above may be altered or modified, and all suchvariations are considered within the scope and spirit of theapplication. Accordingly, the protection sought herein is as set forthin the description. It is apparent that an application with significantadvantages has been described and illustrated. Although the presentapplication is shown in a limited number of forms, it is not limited tojust these forms, but is amenable to various changes and modificationswithout departing from the spirit thereof.

What is claimed is:
 1. A flight control system for controlling theflight of a tiltrotor aircraft while the aircraft is in flight that isat least partially rotor-borne, the aircraft having a fuselage, a wing,and at least two tiltable rotors, each rotor having adjustable-pitchblades controlled by cyclic swashplate controls and collectiveswashplate controls, the flight control system comprising: a first rotorrotatably mounted to the wing; a second rotor rotatably mounted to thewing; software-implemented control laws for automatically tilting therotors in response to the longitudinal-velocity control signal so as toproduce a longitudinal thrust-vector component for controllinglongitudinal velocity of the aircraft; and software-implemented controllaws for automatically actuating the cyclic swashplate controls for eachrotor so as to maintain the fuselage in a desired pitch attitude;wherein the first rotor and the second rotor pivot about the wingbetween a vertical orientation and a horizontal orientation.